low wake design

Discussion in 'Boat Design' started by halfrhovsquared, Jan 9, 2020.

  1. halfrhovsquared
    Joined: Jan 2020
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    halfrhovsquared Junior Member

    Need advice on a personal project design I have in mind. Here are the mission requirements:

    * minimum wake size/height (very important)

    *multi-hull. either a trimaran with small stability sponsons or a catamaran with a rigid deck of some sort

    * powered by small 4-stroke outboard motor that is removable and kept in the trunk of the car when transporting

    * top speed: 15mph

    * car toppable in 2-3 pieces that can be removed by one person and weight less than 80lb each piece and are low drag when mounted on roof rack (favors a trimaran with small sponsons)

    * maximum length of any one piece that can be reasonably car topped is ~22'

    * needs to carry 270 lb of persons plus gear (plus engine and fuel)

    * assume the entire displacement to be 450 lb

    * construction will be stitch-and-glue Okoume with carbon fiber reinforcement and will be light

    * does not have to be overly rugged- will be relatively lightly used on calm water (1 ft high maximum waves with the occasional wake)

    * must have a place to sit/stand in the center to drive the boat plus stability to walk or kneel on the sponsons occasionally (nearly always just driven from middle- only walking on sponsons when docking, etc.)

    * outboard motor should ideally mount behind driver in the middle on a vertical transom or down through a sealed hole in the boat

    Leaning towards a trimaran with sponsons with the motor possibly poking down through a hole to increase the length of the boat. Though a catamaran might work too if I could do some sort of center to mount the motor on but still collapse to be car toppable.

    Does it have to be a planing style hull to go 15mph? How big of 4-stroke outboard kicker do I need (hoping to get away with something like a Yamaha F6LMHA that weighs 61 lb)? How long does the boat have to be to achieve my goals?

    How is the wake size affected by the buttock angle?

    What is the wake size as a percent of maximum hull speed (for example, if I have to go 200% of hull speed to get 15 mph how big is the wake compared to going at 100% hull speed).

    Any idea on simplified criteria for wake size vs speed vs displacement?

    Also, not completely opposed to hydrofoils though would first like to explore a classic design before overly complicating things.

    BTW, promise this is not a homework problem- much too old for that :)

    Ideas?
     
  2. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Welcome to the forum halfrhovsquared :)cool:)

    This is an 'easy' one. In the sense if you wish to have minimal wash, then you need a high length-displacement ratio.
    In the order of 9.0+.
    Length = L
    Displacement = tonne
    LD Ratio = L/D^(1/3)
     
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  3. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Ad Hoc's forumla is for length in meters and displacment in metric tonnes.
    With
    L = length in feet
    D = displacement in pounds
    LD Ratio = 4 * L/D^(1/3)
     
  4. halfrhovsquared
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    halfrhovsquared Junior Member

    Thanks. That works out to about 18' long which makes sense. Idea on my other questions about design style?
     
  5. DCockey
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    DCockey Senior Member

    Longer would reduce wake at higher speeds.

    Design - narrow canoe.
     
  6. Squidly-Diddly
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    Squidly-Diddly Senior Member

    wake box keel - Google Search https://www.google.com/search?client=firefox-b-1-d&sxsrf=ACYBGNSE7GA2Q8iB7NVJIQzrUSO8xuRUXA%3A1578715154241&ei=EkgZXtyiDsXT-gSCzJvQAg&q=+wake+box+keel&oq=+wake+box+keel&gs_l=psy-ab.12..0i8i30.23819.23819..26545...0.2..0.95.95.1......0....1..gws-wiz.......0i71.Jljim7a5Z8Y&ved=0ahUKEwjc8-Cf1PrmAhXFqZ4KHQLmBioQ4dUDCAo

    Box keel boat is probably nice and easy to build due to flat surfaces. Not sure if its one of those things that "work like magic" but only when engineered by someone who knows what they are doing and after tests, etc. Not sure if it would be be good for breaking down into smaller parts.

    IMO you need to change "80lb max pieces" into "80lb max at light end of boat(bow) to get up on roof-rack ROLLER. Having the boat all once piece will make everything else work and reduce overall weight of boat. If you are OK with 22' length then multi-piece seems unnecessary. Re-investigate ways to get a "heavy" one piece boat up on your car. Consider doing it in several steps, maybe start with Forearm Forklifts to get it up on first level step, then just arms to raise bow to about 4-5ft, then use shoulder to get to roof-rack height.

    I think about 18' x 40" box-hull atop a box-keel square stern canoe would be easy to home-build out of thin plywood at under 100lbs, then add a frame for outriggers out of PVC pipe and 6" pool-noodles. IMO you will soon want to omit the outriggers. Box-hull atop a box-keel might lend itself to a two-piece construction, AND possibly allow for making the box-keel a detachable option, with a very viable super-shallow draft basic-boat. Make it so the box-keel can detach and serve as fairly large ama, with enough displacement potential to allow for "walking". I don't see this being done easily on the water without getting wet, but could be an interesting multi-mode boat none the less. A detachable box-keel might be fastened to the hull with 4 metal tabs on front, back and both sides of keel where it meets the hull, and some metal insets on the hull, and four 1/4-20 flat-head machine screws (and a #3 screwdriver)

    My understanding is the box-keel wake reducing magic happens because the box-keel's wake is quashed by the flatness of the main-hull. The wake energy just provides some upward force.

    "plus stability to walk or kneel on the sponsons occasionally (nearly always just driven from middle- only walking on sponsons when docking, etc.)" Not gonna happen with lightweight boat. Putting one foot, and SOME weight out on sponson OK, but unless your 270lb payload is 200lbs of gear and only 70lbs of you I don't see any "walking" on outriggers.
     
  7. philSweet
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    philSweet Senior Member

    Advice: buy a 13 foot jonboat and call it good.

    As a sidenote, the Farrier Trailertri is a just a touch bigger, and weighs 2750 pounds. Can you explain your weight estimate?
     
  8. clmanges
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    clmanges Senior Member

    Please don't give people the idea that they can use PVC pipe as a structural material. It isn't designed to do anything but carry water; not to withstand loading of any sort.
     
  9. clmanges
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    clmanges Senior Member

    I'm afraid you have that backwards. The "basic boat" would have a much deeper draft without the added displacement from the box keel. And without the box keel attached, I'm not sure that what's left will perform very well, either, since that part of the hull isn't designed to be deeply submerged.
     
  10. Squidly-Diddly
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    Squidly-Diddly Senior Member

    too much weight, too much impact res
    PVC is plenty structural. Just because its not stamped and approved don't mean it ain't. PVC is also the Poor Man's Kydex, and can be heated, then shaped and cooled to new forms.
    Here is a niffy 1 1/2" chisel holster I made by heating PVC in oven at 300 for about 5 mins and trying for form with thick gloves. 1st attempt no less. It tries to reform to original shape until doused with cold water. Works great and stands up to jobsite abuse, being sat on, etc. 102 grams. Only improvement is the folded tab could be a big longer if I wanted to clip on belt. My first and only self designed and built kayak used PVC for stringers and they took considerable abuse. I'd trust PVC as more RELIABLE structural material than any wood, since even good quality wood could have weird natural hidden defects. Countless lite trailers, tent frames, tree houses and jungle gyms have been built with PVC (and pvc glue) and I've not heard a single failure "horror story". IIRC this PVC Potato Cannon death was caused by overfilling with gasoline, which coated the victim and burned him to death, NOT failure of PVC structure (doesn't gas dissolve PVC?)Why Potato Cannons Disappeared and Nobody Said Goodbye https://www.vice.com/en_nz/article/43y5qn/how-potato-cannons-disappeared-and-nobody-said-goodbye.
     

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  11. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    Sonova crap! That's a portly boat. For realistic boats in the lightest lowest drag range theoretically possible, take a look at rowing shells. My sweetheart rows every morning in a single she can easily carry alone. It's a one person boat, 22' long. Like a needle on a diet. Obviously you don't want a boat you need to work to balance, but the rowing boat has outriggers that a massively powerful Olympic athlete can't distort. So put your mini hulls on those outriggers. Clip your 1.5 hp outboard to the seat mount.

    Doesn't fully meet your SOR but it's a pointer starting from off the shelf components that are used daily.
     
    Last edited: Jan 11, 2020
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  12. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Imposed or preferred?
    Cruise speed? Same?
    And what are you going to do with the boat?
    Sit and cruise, fish, party, what?
     
  13. JSL
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    JSL Senior Member

    regarding the Displacement/Length ratio. If you are working in U.S./Imperial units it is well explained in the attached by Ted Brewer. Generally, the lower the ratio (number) the less resistance.
     

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  14. philSweet
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    philSweet Senior Member

    Perhaps we could start with something that -

    1. Actually exists.
    2. Can be cartopped
    3. Can handle the prefered motor, not that it is approved to do so.
    4. Isn't too far short of the desired speed.

    "2018 Clipper MacKenzie 16.5 square stern canoe with 2018 6hp 4-stroke Yamaha outboard on the Potomac river. Runs 13 MPH with two 200 lbs men and fishing equipment. Great boat and great motor!"



    Now, where do we need to go from here?
     

  15. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    How about a swamp boat? Flat bottom, planes easily, very little wake. Swamp boat.jpg
     
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